Hierarchical hydrophilic/hydrophobic micro/nanostructures for pushing the limits of critical heat flux

ABSTRACT

A high efficiency heat sink for the cooling of microelectronic devices involves a phase change from liquid fluid to fluid vapor with a vapor quality of 100%. The liquid fluid is provided to an active area that contains fins having micrometer dimension that support a membrane that is nanoporous. The membrane is effectively impermeable to liquid fluid but permeable to fluid vapor. The heat sink provides very high heat flux and coefficient of heat transfer at low mass flux over a broad range of surface superheat temperatures. The heat sink can be constructed of equi-spaced posts that separate liquid microchannels from vapor microchannels that are connected through capillary forced valves formed between adjacent equi-spaced posts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNo. PCT/US2014/070903, filed Dec. 17, 2014, which claims priority toU.S. Provisional Application Ser. No. 61/917,177, filed Dec. 17, 2013,which is hereby incorporated by reference herein in their entireties,including any figures, tables, or drawings.

BACKGROUND OF INVENTION

The primary mode of heat transfer for several decades has beensingle-phase cooling due to its low cost and reliability. Very high heatflux dissipation has become critical for miniaturized electronics withincreased chip power densities. The use of microchannels forsingle-phase cooling has been pursued but is limited by the high chiptemperature and large pumping power required. The traditionalsingle-phase heat sinks are no longer capable of dissipating thegenerated heat in such applications. As a result, introduction ofinnovative thermal management methods is desired to address the demandsof future electronic devices. Phase change heat sinks where boiling of asolvent is carried out has the ability to dissipate large quantities ofheat. Phase change heat transfer that utilizes latent heat ofvaporization is considered to be a solution for removing high heat fluxfrom electronic devices. Research of flow boiling in microchannels hasfocused on the heat transfer and pressure drop performance of flowboiling in microchannels.

Flow instability, low heat transfer coefficient, and flowmisdistribution have resulted in poor performance of two-phase systems.Various techniques have been pursued to avoid high pressure drops orpressure fluctuations. These techniques include artificial nucleationsites, inlet restrictors, and different inlet/outlet configurations.Over the past decades, extensive research efforts have been focused onenhancing boiling heat transfer and mitigating its issues. Most of thesestudies involve modification of the heated surface to enhance surfaceroughness, effective heat transfer area, or active nucleation sites.More recently, wick structures are also employed in microchannels topromote liquid delivery to the heated area and avoid dry-outs. Variousgeometries have been pursued, including: parallel trapezoidal crosssection microchannels; parallel diverging microchannel with artificialnucleation sites; and parallel triangular microchannels. Generally, itis held that the use of inlet restrictors, an increase in systempressure, an increase in channel cross-section, a reduction in thenumber of channels, and a reduction in channel length allow a morestable flow in microchannels. Additional effects have been pursued bythe use of: expanding microchannels for lower pressure drop and walltemperature fluctuation; cross-linked microchannels; square parallelmicrochannels; and microchannel heat sink with structured reentrantcavities. Open microchannels with tapered manifold configuration havebeen shown to simultaneously increase the heat transfer coefficient andthe critical heat flux (CHF), which is the thermal limit of phase changeduring heating where there is a sudden decrease in the efficiency ofheat transfer and overheating of the heating surface. In this manner,relatively high cooling rates without reaching CHF has been observed.However, high cooling rates have only been achieved with high surfacesuperheating, high mass flux, low heat transfer coefficients, and/or lowvapor exit quality. Although improvements in the heat transfercharacteristics of boiling at microscale have been reported, the natureof the boiling process is not altered and, hence, the problemsassociated with boiling could not be eliminated.

The dynamics of heat transfer during nucleation process involves threedifferent sub-mechanisms where, during bubble nucleation, micro-layerevaporation is the most effective mode of heat transfer and candissipate heat fluxes up to 3 times greater than other sub-mechanisms.Flow boiling heat transfer in microchannels displays heat transfer rateswith the thin film evaporation that is greater than the averageconvective boiling processes. However, thin film/micro-layer evaporativemodes of heat transfer occur only for a very short period of time (<5ms) and over small areas; thus the overall boiling heat transfer rate islower than these modes. Dynamic and static instability issues associatedwith boiling at microscale and identified random nucleation, suddengrowth of bubbles and moving evaporating thin film liquid is the causeof these instabilities and whenever liquid-vapor interfaces are rapidlydisrupted, flow instability issues are intensified.

Hence there remains a need to have a heat sink for miniaturizedelectronics with high chip power densities that display high coolingrates, high heat transfer coefficients, low surface superheattemperatures, high vapor exit quality, and superior coefficients ofperformance (COP).

BRIEF SUMMARY

Embodiments of the invention are directed to heat sinks that have aliquid fluid entrance but a vapor fluid exit, where the vapor quality is100%. The heat sink can employ a nanofiber membrane through which vaporreadily passes but liquid is constrained by the nanoporous membrane. Thedevice employs trenches that supply liquid refrigerant to an active areawhere fins support the membrane from which refrigerant exits as a vaporto a condenser. The rate of liquid introduction is readily balanced withthe vapor exit to be of service over an extremely wide range of heatloads. The device employs a new two-phase cooling approach that is basedon phase change over a predefined meniscus. The heat sink utilizesmicro-engineered-structures to form a capillary-controlled meniscusseparating liquid and vapor fields at specific locations. Thisconfiguration maintains stable liquid-vapor interface over a large areato employ thin film evaporation mode of heat transfer while alleviatingflow instabilities.

Embodiments of the invention are directed to two-phase heat sinks thatutilize thin film evaporation over a predefined liquid-vapor interfaceas the main mode of heat transfer. In one embodiment, liquid and vapormicrochannels are separated with arrays of equi-spaced posts, which actas capillary forced valves and limit mixing of liquid and vapor phases.The location and shape of the meniscus can be altered by the devicegeometry, surface characteristics, and liquid pressure at the interface.In this manner, instability issues associated with randomly-movingevaporative interface in boiling can be significantly reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows images of portions of the heat sink: a) an isometric viewof the device's active area; b) scanning electron microscopy (SEM) imageof a top view of the active area; c) a further magnified view of thefins of the active area; and d) the supported membrane, according to anembodiment of the invention.

FIG. 2 shows a plot of heat flux as a function of superheat temperaturefor a heat sink, according to an embodiment of the invention, relativeto that reported for other heat sinks designed for microelectronicdevices, where comparative values for heat sinks that are plotted inFIGS. 2-5 are from:

-   Chen T, Garimella S V. (2006) Measurements and high-speed    visualizations of flow boiling of a dielectric fluid in a silicon    microchannel heat sink. Int J Multiph Flow 32:957-971.-   Kalani A, Kandlikar S (2013) Experimental Investigation of Flow    Boiling Performance of Open Microchannels with Uniform and Tapered    Manifolds (OMM). ASME 2013 Summer Heat Transf. Conf. Minneapolis, pp    1-9.-   Kosar A, Kuo C-J, Peles Y (2006) Suppression of Boiling Flow    Oscillations in Parallel Microchannels by Inlet Restrictors. J Heat    Transfer 128:251.-   Kuo C, Kosar A, Peles Y (2006) Bubble dynamics during boiling in    enhanced surface microchannels. Microelectromechanical Syst J    15:1514-1527.-   Kuo C-J, Peles Y (2007) Local measurement of flow boiling in    structured surface microchannels. Int J Heat Mass Transf    50:4513-4526.-   Li D, Wu G S, Wang W, et al. (2012) Enhancing flow boiling heat    transfer in microchannels for thermal management with    monolithically-integrated silicon nanowires. Nano Lett 12:3385-90.-   Liu D, Garimella S V. (2007) Flow Boiling Heat Transfer in    Microchannels. J Heat Transfer 129:1321.-   Morshed a. K M M, Yang F, Yakut Ali M, et al. (2012) Enhanced flow    boiling in a microchannel with integration of nanowires. Appl Therm    Eng 32:68-75.-   Qu W, Mudawar I (2003) Flow boiling heat transfer in two-phase    micro-channel heat sinks—I. Experimental investigation and    assessment of correlation methods. Int J Heat Mass Transf    46:2755-2771.-   Steinke M E, Kandlikar S G (2004) An Experimental Investigation of    Flow Boiling Characteristics of Water in Parallel Microchannels. J    Heat Transfer 126:518.-   Wang G, Cheng P, Bergles a. E (2008) Effects of inlet/outlet    configurations on flow boiling instability in parallel    microchannels. Int J Heat Mass Transf 51:2267-2281.

FIG. 3 shows a plot of heat flux as a function of mass flux for a heatsink, according to an embodiment of the invention, relative to thatreported for other heat sinks designed for microelectronic devices.

FIG. 4 shows a plot of heat transfer coefficient as a function of exitvapor quality for a heat sink, according to an embodiment of theinvention, relative to that reported for other heat sinks designed formicroelectronic devices.

FIG. 5 shows a plot of the coefficient of performance as a function ofheat flux for a heat sink, according to an embodiment of the invention,relative to that reported for other heat sinks designed formicroelectronic devices.

FIG. 6 is a drawing of heat sinks' liquid and vapor microchannels thatare separated by arrays of equi-spaced posts that form capillary forcedvalves between the liquid and vapor microchannels, according to anembodiment of the invention.

FIG. 7 is a schematic of a unit cell of the embedded evaporative heatsink formed at the capillary forced valves formed between themicro-posts as shown in FIG. 6, according to an embodiment of theinvention.

FIG. 8 is a schematic of the computational domain of the unit cell ofFIG. 7 which is discretized by structured hexahedron meshes forcomputation of the characteristics of a heat sink, according to anembodiment of the invention.

FIG. 9A shows a photograph of the exemplary heat sink, according to anembodiment of the invention.

FIG. 9B shows a scanning electron microscopy (SEM) image of the liquidand vapor microchannels separated by capillary forced valves, accordingto an embodiment of the invention.

FIG. 10 is a schematic of a flow loop used to evaluate the exemplaryheat sink of FIG. 9, according to an embodiment of the invention.

FIG. 11A is an illustration of the anticipated effect of liquid pressureon the meniscus that forms the capillary forced valve as a top view atthe middle of a microchannel, z=75 μm.

FIG. 11B is a numerical simulation of an isometric view, whereΔP3>ΔP2>ΔP1.

FIG. 12A is a drawing of the effect of supplied pressure on the meniscusillustrating a view at a plane at the middle of microchannel, z=75 μm.

FIG. 12B is a drawing of the effect of supplied pressure on the meniscusillustrating an isometric view over the height of the posts whereΔP3>ΔP2>ΔP1.

FIG. 13A illustrates the effect of liquid pressure on the meniscus andtemperature distribution (ΔP₃>ΔP₂>ΔP₁).

FIG. 13B illustrates local wall heat flux variations for the device ofFIG. 13A over the side wall of the micro-pillars with pressure,according to an embodiment of the invention.

FIG. 14A is a plot of wall heat flux variations over the side wall ofthe fins as a function of supplied pressure (z=50 μm).

FIG. 14B is a plot of local evaporative mass flux over the liquid-vaporinterface as a function of supplied pressure (z=50 μm).

FIG. 15A is a plot of experimental heat flux data as a function of wallsuperheat at different supplied liquid pressures.

FIG. 15B is a plot of calculated heat transfer coefficients vs. wallsuperheat at different supplied liquid pressures for the heat sinkdevice of FIG. 9A, according to an embodiment of the invention.

FIG. 16 shows numerical temperature contour maps of the liquid at arelatively low liquid pressure, left, and a relatively high liquidpressure, right, for a heat sink device of FIG. 9A, according to anembodiment of the invention.

FIG. 17 shows cross-section drawings of two different configurationsdiffering by the surface area of the liquid-vapor microchannel due tothe geometry of the spacers separating the heat sink from the membrane,according to embodiments of the invention.

FIG. 18 shows a plot of the heat flux per the pressure drop in themicrochannel versus the pressure drop between the liquid and vapor waterfor the two configurations of FIG. 17, according to an embodiment of theinvention, for the three membranes of Table 2 with respect to thetheoretical limit (broad solid line) for each combination, illustratingthe effect of the membrane properties.

FIG. 19 shows a plot of the heat flux versus the pressure drop in themicrochannel for the two configurations of FIG. 17, according to anembodiment of the invention for the three membranes of Table 2 withrespect to the theoretical limit (broad solid line) for eachcombination.

FIG. 20 shows a plot of the heat flux per the pressure drop in themicrochannel versus the pressure drop between the liquid and vapor waterfor microstructure 1 of Table 3, according to an embodiment of theinvention, and a plane surface with respect to the theoretical limit(broad solid line) for each combination, illustrating the effect of themembrane area.

FIG. 21 shows a plot of the heat flux versus the pressure drop in themicrochannel between the liquid and vapor water for microstructure 1 ofTable 3, according to an embodiment of the invention, and a planesurface with respect to the theoretical limit (broad solid line) foreach combination.

FIG. 22 shows a plot of the heat flux per the pressure drop in themicrochannel versus the pressure drop between the liquid and vapor waterfor microstructures of Table 3, according to an embodiment of theinvention, with respect to the theoretical limit (broad solid line) foreach combination, illustrating the effect of the wickability of themicrostructures.

FIG. 23 shows a plot of the heat flux versus the pressure drop in themicrochannel between the liquid and vapor water for microstructures ofTable 3, according to an embodiment of the invention, with respect tothe theoretical limit (broad solid line) for each combination.

FIG. 24 shows a plot of the heat flux per the pressure drop in themicrochannel versus the pressure drop between the liquid and vapor waterfor microstructures of Table 3, according to an embodiment of theinvention, with respect to the theoretical limit (broad solid line) foreach combination, illustrating the effect of the surface roughness ofthe microstructures.

FIG. 25 shows a plot of the heat flux versus the pressure drop in themicrochannel between the liquid and vapor water for microstructures ofTable 3, according to an embodiment of the invention, with respect tothe theoretical limit (broad solid line) for each combination.

FIG. 26 shows a plot of the heat flux per the pressure drop in themicrochannel versus the pressure drop between the liquid and vapor waterfor microstructures of Table 3, according to an embodiment of theinvention, with respect to the theoretical limit (broad solid line) foreach combination, illustrating the combined effect of the surfaceroughness of the microstructures and membrane area.

FIG. 27 shows a plot of the performance relative to theoretical limitsof the heat sink with respect to heat flux versus the pressure drop forthe microstructures of Table 3, according to an embodiment of theinvention.

FIG. 28 and FIG. 29 show a plot of the performance relative totheoretical limits of the heat sink with respect to the microstructureswith heat flux per the pressure drop in the microchannel versus thepressure drop, according to an embodiment of the invention.

DETAILED DISCLOSURE

Embodiments of the invention are directed to heat sinks that require,but are not limited to, a single liquid fluid inlet and have exclusivelya vapor fluid outlet through a membrane. The liquid fluid portion of aheat sink, according to an embodiment of the invention, is illustratedin FIG. 1. The fluid can be any fluid, ranging from water throughhydrofluorocarbon refrigerants, for example: trifluoromethane,difluoromethane, fluoromethane, pentafluoroethane, pentafluorodimethylether, 1,1,2,2-tetrafluoroethane, 1,1,1,2-tetrafluoroethane,bis(difluoromethyl)ether, 1,1,2-trifluoroethane, 1,1,1-trifluoroethane,methyl trifluoromethyl ether, 2,2,2-trifluoroethyl methyl ether,1,2-difluoroethane, 1,1-difluoroethane, fluoroethane,1,1,2,2,3,3,3-heptafluoropropane, trifluoromethyl1,1,2,2-tetrafluoroethyl ether, 1,1,1,2,3,3,3-heptafluoropropane,trifluoromethyl 1,2,2,2-tetrafluoroethyl ether,1,1,1,2,2,3-hexafluoropropane, 1,1,1,2,3,3-hexafluoropropane,1,1,1,3,3,3-hexafluoropropane, 1,2,2,2-tetrafluoroethyl difluoromethylether, hexafluoropropane, 1,1,2,2,3-pentafluoropropane,pentafluoropropane, 1,1,2,3,3-pentafluoropropane,1,1,1,2,3-pentafluoropropane, 1,1,1,3,3-pentafluoropropane, methylpentafluoroethyl ether, difluoromethyl 2,2,2-trifluoroethyl ether,difluoromethyl 1,1,2-trifluoroethyl ether, 1,1,2,2-tetrafluoropropane,methyl 1,1,2,2-tetrafluoroethyl ether, trifluoropropane, or anyfluorocarbon or hydrofluorocarbon. In general, flammable fluids areavoided, but there is no limitation to the fluid as long as the boilingpoint is about 100° C. or less. In an embodiment of the invention, themembrane is a nanostructured membrane that is of a hydrophobic material.Hydrophobic materials are defined as any material upon which a waterdroplet displays a contact angle of at least 90 degrees. For example, anexemplary membrane is a polytetrafluoroethylene (PTFE) nanofibermembrane. Any other material can be used as long as the material ishydrophobic as defined by the water contact angle.

The membrane is supported by fins, which are supports that extendperpendicularly from the base of the heat sink in the active area of theheat sink, where the base is the surface through which heat istransferred from the microelectronic device to the fluid through theexposed base surface and the surfaces of the fins. The fins can have anygeometry that allows the flow of a refrigerant between the fins. Thefins permit fluid to flow from multiple sides and are not restricted toflow from one dimension, as is common to current microchannel heat sinkdevices. The fins have dimensions of about 1 to about 200 micrometer incross-section and have a height that is equal to or greater than thecross-section and can be about 250 micrometers or less. The fins can beof any shape; for example, the fins can be square pillars in shape,cylinders in shape, or any polygon in shape that permits relativeunimpeded flow of fluid into the active area. The fins can be patternedby voids or other features on their surfaces to increase surface area,promote nucleation of vapor, or perform any other desirable action. Thefins support the membrane and can transfer heat from the base of theheat sink. The heat sink base and fins can be selected independentlyfrom any material with thermal conductivity greater than about 10 W/mK.In some embodiments of the invention, the base and the fins are of thesame material. In an exemplary embodiment, the heat sink can be siliconwith a thermal conductivity of 149 W/mK. Other materials that can beemployed include metals and composites that include a filler of highthermal conductivity, such as metal fibers, carbon nanotubes, orgraphene flakes.

In embodiments of the invention, the central portion of the heat sink isthe active area that contains the fins. The active area is surrounded bytrenches that fill with fluid and supply the fluid to the active area.The trenches can have the same depth as the fins, sharing a common baseas that of the active area, or can be of other depths that promoteregular even flow of fluid into the active area. The trenches cancontain a volume of fluid that is similar to the volume of fluid thatcan be contained within the active area. For example, the trenches cancontain about 70 to about 150 percent of the fluid volume as the activearea. The active area need not be square, as illustrated in FIG. 1, butcan be circular or of any other geometry. Generally, although notnecessarily, the geometry of the active area is a regular geometry, forexample, a regular polygon, and generally, but not necessarily, thetrenches are situated regularly about the active area.

In embodiments of the invention, a second portion, not illustrated inFIG. 1, is situated on the face distal to the base of the heat sink. Thesecond portion covers at least the active area of the first portion anddirects the vapor fluid to a condenser where the vapor reverts to aliquid for reintroduction into the heat sink. The condenser ismaintained at a temperature desirable to return the vapor to the liquidphase at a temperature equal to or below the temperature of the liquidintroduced to the heat sink. Depending on the fluid employed and thepressure maintained within the system, the temperature can besub-ambient to temperatures higher than the boiling point of the fluidat ambient pressures. The pressures can be sub-ambient to multipleatmospheres, depending upon the heat sink material's ability to safelymaintain that pressure, where, for example, for a metal heat sink,pressures in excess of 100 psi are possible. The liquid fluid can returnto the heat sink from the condenser by the force of gravity or asprovided by a pump. The condenser or any fluid reservoir downstream ofthe condenser can be used to feed one or more heat sinks.

The exemplary heat sink, of the design indicated in FIG. 1, can providevastly superior performance to that of state of the art heat sinks formicroelectronic devices. As can be seen in FIG. 2, heat flux is plottedas a function of superheat temperature and is compared to prior art heatsinks reported in the literature. The trend indicates that the heat fluxof the instant invention is a strong function of the surface superheattemperature, which, unlike that displayed by the majority of prior artdevices that employ uniform micro channels, does not approach a plateauheat flux over the range of temperatures examined. Although not to belimited by mechanism, the performance of the heat sink somewhatresembles that of impingement cooling systems.

FIG. 3 illustrates the heat flux dependence on mass flux of the device.Again, in comparison to other state of the art heat sinks, a superiorheat flux is achieved at a much lower mass flux, being more than anorder of magnitude lower than that previously reported. As the systemhas an unprecedented 100% vapor quality, a dramatically higher heattransfer coefficient is achievable. When compared to the previousmaximum vapor quality device, the heat transfer coefficient is about anorder of magnitude higher for the heat sink, according to an embodimentof the invention.

FIG. 4 shows a plot of the heat sink's coefficient of performance (COP)where: COP=VΔP, where V is the volumetric flow rate and ΔP is thepressure drop. The COP is superior to state of the art heat sinks bymore than an order of magnitude.

Hence, a heat sink, according to an exemplary embodiment of theinvention, has been constructed from silicon with a PTFE membrane, inthe manner illustrated in FIG. 1, that displays a cooling rate of 383W/cm² and a heat transfer coefficient of 264 kW/m²K with an exit vaporquality of 100% at a mass flux of 28.4 kg/m²s and a surface superheattemperature of 23.4° C. This compares favorably with the state of theart cooling rate of 370 W/cm² that requires a surface superheattemperature of 98.4° C., or displays a lesser, but still relativelyhigh, heat flux of 283 W/cm² and heat transfer coefficient of 218kW/m²K, but requires a high mass flux of 372 kg/m²s with a vapor exitquality of about 10%, Kalani and Kandlikar (2013). The exemplary heatsink, according to an embodiment of the invention, displays anunprecedented coefficient of performance (COP) that is an order ofmagnitude higher than any prior heat sink for microelectronic devices.

In an embodiment of the invention, thin film evaporation over apredefined liquid-vapor interface as the main mode of heat transfer byemploying liquid and vapor microchannels that are separated with arraysof equi-spaced posts, which act as capillary forced valves and limitmixing of liquid and vapor phases, is shown in FIG. 6. As shown in FIG.6, the meniscus plane is normal to the heated surface and the totalevaporation surface depends on the height of the micro-posts.

FIG. 7 shows a schematic of a unit cell of the embedded evaporative heatsink formed at the capillary forced valves formed between themicro-posts. In an exemplary embodiment, the heat sink device isfabricated on a silicon wafer, with dimensions given in Table 1, below,where these dimensions are selected to be within the capabilities andlimits that exist in standard state of the art micro-fabricationmethods, such as Deep Reactive Ion Etching (DRIE).

TABLE 1 Geometrical Dimensions of micro-fabricated device ChannelHeight, H (μm) 150 Liquid Channel Width, S1 (μm) 30 Fin Width, S2 (μm)30 Vapor Channel Width, S3 (μm) 30 Gap Width, 2 × W1 (μm) 15 Center toCenter Pillar Spacing, 2 × W2 (μm) 35

Thermohydraulic characteristics of the proposed heat sink are computedby a set of numerical simulations performed for the unit cell domainshown in FIG. 7 for different working conditions. The computationaldomain is discretized by structured hexahedron meshes, as illustrated inFIG. 8. In simulations, liquid and vapor phases are separated by aninterface (ABCDEA plane in FIG. 8) across which the phase-change occurs.The interface temperature is fixed at the equilibrium saturationtemperature corresponding to the system pressure. The boiling masstransfer rate at the liquid-vapor interface is equal to the jumpcondition on the thermal flux divided by the latent heat ofvaporization:

$\begin{matrix}{{{\overset{.}{m}}_{evap} = {\left( \frac{{{- k_{liq}}{{\nabla\; T} \cdot \overset{\rightarrow}{n}}} + {k_{vap}{{\nabla\; T} \cdot \overset{\rightarrow}{n}}}}{h_{fg}} \right)\frac{A_{interface}}{\forall_{cell}}}},} & \lbrack 1\rbrack\end{matrix}$where n is interface normal vector, A_(interface) is area of interfacelocated within the computational cell and ∀_(cell) is the cell volume,as indicated in FIG. 8. To accurately estimate the spatial gradients atthe interface, a Level-Set method is used, which is an interfacetracking scheme with a smooth and continuous nature. In Level-Setmethod, the interface motion is captured by a convection equation for aLevel-Set Function, ϕ, which zero level set separates the vapor phase(ϕ>0) from the liquid phase (ϕ<0):

$\begin{matrix}{{\frac{\partial\phi}{\partial t} + {\overset{\rightarrow}{v} \cdot {\nabla\phi}}} = 0.} & \lbrack 2\rbrack\end{matrix}$The level-set method is utilized only to compute the spatial gradientsin calculations of evaporation rate and surface tension forces at theinterface. To track the liquid and vapor volume fractions in each cellover the entire computational domain, a volume-conserved technique, aVolume of Fluid (VOF) method, has been employed. In this method, aconvection equation is used to calculate the liquid volume fraction, αl,as:

$\begin{matrix}{{{\frac{\partial}{\partial t}\left( {\alpha_{l}\rho_{l}} \right)} + {\overset{\rightarrow}{v} \cdot {\nabla\left( {\alpha_{l}\rho_{l}} \right)}}} = {{\overset{.}{m}}_{evap}.}} & \lbrack 3\rbrack\end{matrix}$Volume conservation in each cell dictates the vapor volume fraction tobe as α_(v)=1−α_(l). Since, the volume fraction values do not uniquelyidentify the interface between the phases, an interface reconstructingscheme is inevoked. A piecewise linear geometric reconstruction schemeavailable in FLUENT, (User's Guide for ANSYS FLUENT 12, Fluent Inc.,2009) is employed to update the volume fraction values in each cellthrough a reconstruction step followed by a propagation step. In thereconstruction step, the orientation of the segment that divides eachinto the vapor and liquid parts is calculated. In the propagation step,the flux of volume of a particular fluid through a mixture cell faceduring a time step is computed and the volume of fraction field at thenew time is updated. In the exemplary heat sink device using a lengthscales of 10-150 μm, the effect of gravitational forces is found to benegligible with respect to surface tension forces Bond number,Bo=ρgL2/σ˜10−5−10−3) and is not taken into account in the numericalsimulation. Surface tension forces are considered using a continuumsurface force (CSF) model.

The surface tension force can be converted into a volumetric force usingthe divergence theorem:

$\begin{matrix}{F_{vol} = \frac{2{\sigma\rho\kappa}{\nabla\alpha_{l}}}{\left( {\rho_{l} + \rho_{v}} \right)}} & \lbrack 4\rbrack\end{matrix}$where σ is the surface tension, ρ is the volume-fraction-averageddensity (ρ=α_(v)ρ_(v)+α_(l)ρ_(l)) in the computational cell and κ is theinterface curvature calculated as the divergence of the normal vectoras:

$\begin{matrix}{\kappa = {\nabla{\cdot {\frac{\nabla\alpha_{l}}{\left| {\nabla\alpha_{l}} \right|}.}}}} & \lbrack 5\rbrack\end{matrix}$This volumetric force then becomes a source for the momentum equation asfollows:

$\begin{matrix}{{{\frac{\partial}{\partial t}\left( {\rho\overset{\rightarrow}{v}} \right)} + {\nabla{\cdot \left( {\rho\overset{\rightarrow}{v}\overset{\rightarrow}{v}} \right)}}} = {{- {\nabla p}} + {\nabla{\cdot \left\lbrack {\mu\left( {\nabla\overset{\rightarrow}{v}} \right)} \right\rbrack}} + F_{vol}}} & \lbrack 6\rbrack\end{matrix}$The finite-volume based solver, FLUENT, is employed to solve the presentmultiphase microscale problem. The evaporation model is implemented viaa User-Defined Function (UDF), written in C code and is capable ofparallel computing. In the UDF, first, computational cells located atthe interface are detected. The interfacial computational cell has atleast one node with a different sign for the level set function comparedto the other nodes. Then, the interface plane (ABCDA plane in FIG. 8)and its normal direction (n) are determined. Subsequently, temperaturegradient component normal to the interface (∇T.n) and area of theinterface within the cells are calculated. Finally, the boiling masstransfer rate of the interface cells (eq. [1]) is computed. The accuracyof the UDF is evaluated by the experimental results, below.

Boundary conditions used in the simulations are provided in FIG. 7.Liquid enters the device at a constant temperature and pressure. Thefluid temperature at the topside of the field is at saturationtemperature, while the top surface of the solid is assumed to be a freeconvection wall. Symmetric and adiabatic boundary conditions are shownin the FIG. 7. Vapor exits the device at a constant pressure. The flowfield is discretized by a uniform computational grid. The griddimensions of the computational domain were 0.2 μm in all threedirections. A grid independence study ensured that the computationalresults are independent of the grid size. The maximum error in thecalculations of local wall heat flux and evaporative mass flux werefound to be less than 3% when the grid size was reduced by a factor of2. Simulations were performed using UF NESL Sandy-Bridge cluster, whichis a high performance server having computing nodes of hexa core IntelXeon E5-2620 processors at 2 GHz and 32 GiB of RAM, where allsimulations were run with 96 parallel cores.

FIG. 9A shows SEM micrographs of the micro-fabricated heat sink. Thereare two types of microchannels inside the heat sink, as indicated inFIG. 9B: channels with an open inlet port (liquid channels) and channelswith blocked inlets (vapor channels). As the working liquid flows intothe liquid microchannels, the capillary force acting between twoconsecutive posts forms a meniscus between the pillars, which stops theliquid from entering into the vapor channel. Geometrical dimensions ofthe exemplary micro-fabricated device are presented in Table 1.

Device performance was tested in a custom made vacuum chamber in whichthe pressure and temperature can be controlled, as shown schematicallyin FIG. 10. The selected cooling liquid, distilled water, is deliveredto the heat sink through a piezoelectric micropump (Model MP6,manufactured by Bartels Mikrotechnik GmbH), with two Vacuum Setrapressure transducers (Setra 730) for monitoring the pressures inside thechamber and the heat sink. An Agilent data acquisition system was usedfor recording the pressure and temperature data of the chamber and thedevice. The water vapor generated during the device operation wascondensed by a thermo electric cooler (TEC) and returns back to thewater reservoir.

To accurately measure the surface heat flux, the heat transfer loss ofthe device was measured as a function of its surface temperature. Theheat was dissipated to the ambient and to the wiring system thatconnects the device heaters and sensors to a set of terminals. Ameasurement was conducted while the heat sink was dry and the energysupplied to the device to maintain it at a set temperature is consideredto be its heat loss.

Hydraulic Characteristics:

Under steady state condition, liquid flows in the main channels andforms a meniscus between the gaps of the two adjacent microposts, in themanner illustrated in FIG. 6. The liquid-vapor interface ischaracterized by surface tension and pressure of supplied liquid.Namely, at equilibrium, the hydrostatic pressure acting on the meniscusshould equal friction and capillary forces. At the interface, theYoung-Laplace equation:

$\begin{matrix}{{{\Delta\; P} = {{P_{l} - P_{v}} = \frac{2\sigma_{f}\mspace{14mu}\cos\mspace{14mu}\theta_{A}}{R}}},} & \lbrack 7\rbrack\end{matrix}$where σ_(f) is surface tension of water, θ_(A) is the advancing contactangle, R is the radius of curvature and P_(l) and P_(v) denote liquidand vapor pressure, respectively.

In order to study the flow characteristics in the heat sink, a set ofvisualization experiments were conducted. A transparent MEMS device withsimilar microstructure was fabricated in Polydimethylsiloxane (PDMS),using soft-lithography techniques and was then covered with a thin PDMSmembrane. Tests were conducted at different liquid pressures toinvestigate their effect on the shape and position of the meniscus.Images were captured from the top-side of the device and at the middleheight of the microchannel (z=75 μm). At low pressure, the interfacecurvature is almost zero and surface friction force acting on theinterface balances the pressure of the liquid and prevents liquid fromflowing into the gap. Increasing the pressure, as illustrated in theFIG. 11A, increases the curvature of meniscus and expands the contactarea between liquid and side walls of the post. The meniscus, which isstill pinned to the top and bottom walls, starts to curve and moveforward to compensate for the increased pressure. Further increase inthe pressure will result in the complete progress of the meniscus untilit is pinned at the end of the gap, as illustrated in FIG. 11A. The sametrend in the advancement of the meniscus by increasing the liquidpressure is also captured in the simulation results and illustrated inFIG. 11B. However, slight differences in the shape and the position ofthe interface are can occur and to visualize these differences, as inFIG. 11A, a constant contact angle measured by the static sessile dropmethod is assumed and the sidewall scallop pattern formed in DeepReactive Ion Etching (DRIE) process affects the surface flatness of themicropillars, which differs from perfectly flat side walls that areassumed for the numerical simulations. When the pressure reaches acritical value (burst pressure), increasing the curvature (i.e.,reducing the radius of curvature) is insufficient to cancel the excesspressure force and the meniscus breaks to allow the liquid to flow intothe vapor channel. Use above the critical pressure is undesirable sinceit eliminates the liquid-vapor interface and therefore is detrimental tothe performance of the heat sink device.

FIGS. 12A and 12B show the steady state temperature field of the liquidand vapor phase as well as the particle path in the unit cell at lowpressure. Liquid heats as it comes close to the fins and evaporates atthe interface. As can be seen from particle path in FIG. 12A, higherevaporation rates occurs in the vicinity of the wall, where asignificant temperature gradient exists between the wall and interface.Non-uniform evaporation rate over the interface causes the liquid tohave high velocity near the wall and low velocity in the middle andtherefore imposes vortices in the flow pattern near the interface. Theparticle path for a 3D meniscus is shown in FIG. 12B, which illustratesthat vortices occur over the entire meniscus to ensure delivery ofliquid to the high evaporation regions.

With the increase in the liquid pressure, the cooling liquid will flowfurther inside the gap and bring more liquid in contact with the sidewalls of the posts. Modeling assumes that the flow conditions aresimilar to flow between two walls with constant temperature. The smallhydraulic diameter of the gap corresponds to significant heat transferrate between the wall and liquid and therefore liquid temperature canrise quickly, as suggested in FIG. 13A. The variation of wall heat fluxon the side wall of fins is illustrated in FIG. 13B. Three distinctregions can be identified as the liquid comes in contact with the fins:Region (A-B) where micro-convective heat transfer dictates the mechanismof heat transfer between the solid and liquid phases, where the heattransfer rate is significant at the entrance, an entry length effect,and decreases as the liquid advances through the gap; Region (B-C) inthe vicinity of the interface, where conduction is the major mode ofheat transfer to remove heat from the fins with conduction heat transferincreasing as the distance between the wall and meniscus reduces andreaches a maximum at the interface at point C; and Region (C-D) in thevapor phase domain where heat transfer is significantly lower comparedto the liquid phase because of lower thermal conductivity and density ofwater vapor.

FIG. 14A shows the variation of the heat flux over the side wall of thefins at different pressures. At low pressure, wall heat flux starts froma maximum value very close to the inlet of the gap and decreasesrapidly. This results because, at low pressure, the meniscus is formedvery close to the gap entrance and, therefore, regions A-B and B-C willoverlap and result in relatively high wall heat flux at the inlet, whichquickly reduces as vapor comes in contact with the wall. Increasing thepressure brings the liquid further inside the gap and the aforementionedregions can clearly be identified. Further incursion into the gap willincrease the temperature of liquid and results in higher conduction heattransfer from wall to interface.

Local evaporative mass flux over the liquid-vapor interface at differentsupplied liquid pressure is depicted in FIG. 14B. High evaporative massflux in the vicinity of the micro-pillars declines at the center ofinterface where a lower temperature gradient exists. As the suppliedliquid pressure increases, the gradient of the mass flux drops. Thereason for this relationship is apparent from FIG. 13A in which a moreuniform temperature field within the micro-pillars is obtained at higherliquid pressure. So as the supplied liquid pressure increases, theuniformity of evaporative mass flux over the entire interface increases.The average evaporative mass flux increases with pressure due to anintensified heat transfer characteristics of the liquid encapsulatedbetween the micro-pillars at elevated supplied pressure. Evaporativemass flux, as plotted in FIG. 14B, demonstrates that the heat sink isnot limited by constraints that are common to other evaporative coolingsystems, where most of all evaporation occurs from a small area, whichdecreases overall thermal performance. The heat sink devices accordingto embodiments of the invention show higher and more uniform evaporationrates over the entire liquid-vapor interface.

Experimentally observed effects of the supplied inlet pressure on thethermal performance of the heat sink device of FIG. 9A, as a function ofsuperheat temperature, is shown in FIG. 15A. Increasing the liquidpressure intensifies the liquid temperature gradient at the meniscus,which leads to an increase in the heat flux dissipated by the device ata constant wall superheat temperature as can be seen in FIG. 15A. FIG.15A provides heat flux data of the numerical simulation withexperimental results, showing agreement within 17%. The differencebetween the numerical and experimental results is consistent with: anon-uniform pressure distribution within the device that varies frommicro-pillar to micro-pillar because of a pressure drop inside theliquid channels as opposed to the constant liquid pressure used in thecomputations; assumption of sidewall flatness of the micro-pillars insimulations as opposed to a scallop patterned wall formed by the DeepReactive Ion Etching (DRIE) process during fabrication; assumption of aconstant contact angle in simulation rather than a contact angle that isdynamically varying between advancing and receding angles; and theslight variation in the experimental local surface temperature of theheat sink observed in experiments but estimated by a constantaveraged-temperature in the numerical simulations.

FIG. 15B shows heat transfer coefficients versus the surface temperatureat different liquid inlet pressures. The heat transfer coefficientincreases with increasing liquid pressure and decreases as the surfacetemperature rises. As boiling starts inside the device, the stableliquid-vapor interface breaks and the heat transfer coefficient drops asindicated in FIG. 15B.

Experimental results presented in FIGS. 15A and 15B illustrate thatincreasing the liquid pressure delays the onset of nucleation boilingtemperature in the heat sink. To explain this behavior, a set ofnumerical simulations were conducted based on the experimentalconditions to explore the region that has the highest probability fortriggering the bubble nucleation process, a region with highesttemperature and the lowest heat transfer coefficient. The simulationssuggest that the region of highest probability of bubble nucleation islocated farthest from the interface and close to the center line of theliquid channel. FIG. 16 illustrates the liquid temperature contours of avertical plane intersecting the center line of the liquid channel at twodifferent inlet liquid pressures. As shown in FIG. 16, increasing theliquid pressure results in a lower bulk temperature, which suppressesbubble nucleation in the main channel.

As shown in FIG. 17 the geometry of the profile of the vapormicrochannels can be modified to increase the membrane area relative tothe area of the heat sink surface. The heat sink can be any heatconductor, for example, but not limited to, a metal or metal alloy, forexample, but not limited to, copper, stainless steel, or brass. A spacercan define the walls from the heat sink to the supported membrane, andthe shape defined by the spacer defines the area ratio of the membraneand the heat sink. The spacer can be of any material, and can be a goodthermal conductor, such as a metal, or can be a thermal insulator, forexample, but not limited to, a rubber or plastic, for example, but notlimited to, silicone or other resin. The membrane can be a hydrophobicor hydrophilic membrane, although hydrophilic membranes can give veryhigh performances, pore sizes can vary as long as the membrane resistsliquid flow through the membrane. The combination of the pore size andmembrane material relationship defines the permeability, for example,the permeability of water is indicated below in Table 2, where thehighest permeability is for the large pore size hydrophilic membrane,Versapor®.

TABLE 2 Membranes Employed in the Heat Sinks for the Data Illustrated inFIGS. 17-27 Pore size Permeability Membrane Material (μm) (l/hr/cm²/kPa)#1 Polyethersulfone 8.0 0.2 (PES) #2 Polytetrafluoroethylene 5 5 (PTFE)#3 acrylic copolymer 10 20 (Versapor ®)

In addition to the nature of the materials and the relative area of themembrane and heat sink, the microstructure of the vapor microchannelsdefines the performance of the heat sinks, according to embodiments ofthe invention. Different microstructures are summarized in Table 3,below.

TABLE 3 Microstructure Parameters for the Heat Sinks for the DataIllustrated in FIGS. 17-27 Fin Spacing, Side, Fin Height, WickabilitySurface Device s (μm) d (μm) h (μm) (cm/s) Roughness, r #1 75 50 50 0.061.64 #2 200 100 150 0.18 1.67 #3 300 50 325 0.31 1.56 #4 200 150 5000.30 3.45

As can be seen in Table 3 and FIGS. 17-27, structures that promote goodwickability and surface roughness give excellent results. A recordcooling capacity of 1700 W/cm² has been achieved as illustrated in FIGS.26 and 27.

All publications referred to or cited herein are incorporated byreference in their entirety, including all FIG.s and tables, to theextent they are not inconsistent with the explicit teachings of thisspecification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

We claim:
 1. A phase change heat sink, comprising: a reservoir definedby a first surface spaced a distance from a second surface, thereservoir being accessible through a liquid fluid inlet, at least one ofthe first surface and the second surface comprising a membrane; and anactive area comprising a plurality of solid features extending from abase, the plurality of solid features defining an interstitial spaceoperable to exert capillary forces on a fluid, the active area beingaccessible through said liquid fluid inlet, wherein: a liquid fluid,communicated through the liquid fluid inlet and into the reservoir, isfully vaporized into a vapor fluid in response to absorbing heat; andsaid vapor fluid is communicated through said membrane and out of thephase change heat sink, said membrane being permeable to said vaporizedvapor fluid and impermeable to said liquid fluid.
 2. The phase changeheat sink according to claim 1, wherein said reservoir comprises aliquid microchannel for said liquid fluid and said plurality of solidfeatures extending from said base comprises an array of posts, andwherein said active area includes a vapor microchannel separated fromsaid liquid microchannel by said posts.
 3. The phase change heat sinkaccording to claim 1, wherein said liquid fluid is water.
 4. The phasechange heat sink according to claim 1, wherein said liquid fluid is afluorocarbon or a hydrofluorocarbon.
 5. The phase change heat sinkaccording to claim 1, wherein said plurality of solid features havecross-sections of 1 to 200 micrometers.
 6. The phase change heat sinkaccording to claim 1, wherein said membrane is a hydrophobic membrane.7. The phase change heat sink according to claim 1, wherein saidmembrane is nanoporous or microporous.
 8. A method to cool amicroelectronic device, comprising: contacting a microelectronic deviceto at least one phase change heat sink according to claim 1; providingsaid liquid fluid to said heat sink; absorbing heat by vaporizing saidliquid fluid to said vapor fluid; passing said vapor fluid through saidmembrane of said heat sink; condensing vapor fluid at a condenser toreform said liquid fluid; and re-providing said liquid fluid to saidheat sink, wherein the vapor quality of said vapor fluid is 100%.
 9. Themethod of claim 8, wherein said providing is performed with a pump. 10.The method of claim 8, wherein said vaporizing said liquid fluid to saidvapor fluid occurs between a liquid microchannel for said liquid fluidand a vapor microchannel.
 11. A phase change heat sink, comprising: afluid reservoir defined by a first surface positioned a distance from asecond surface, the fluid reservoir being accessible by a liquid fluidthrough a liquid inlet, at least one of the first surface and the secondsurface comprising a membrane permeable to a vapor fluid and impermeableto said liquid fluid; and an active area comprising a plurality of solidfeatures extending from a base, the plurality of solid featuresdimensioned and configured to exert capillary forces on the liquidfluid, wherein the liquid fluid is fully vaporized into the vapor fluidin response to the liquid fluid absorbing heat.
 12. The phase changeheat sink according to claim 11, wherein said fluid reservoir comprisesa liquid microchannel for said liquid fluid and said plurality of solidfeatures extending from the base comprises an array of posts, andwherein said active area includes a vapor microchannel separated fromsaid liquid microchannel by said array of posts.
 13. The phase changeheat sink according to claim 11, wherein said liquid fluid is water. 14.The phase change heat sink according to claim 11, wherein said liquidfluid is an alcohol, a fluorocarbon or a hydrofluorocarbon.
 15. Thephase change heat sink according to claim 11, wherein said plurality ofsolid features have cross-sections of 1 to 200 micrometers.
 16. Thephase change heat sink according to claim 11, wherein said membrane is ahydrophobic membrane.
 17. The phase change heat sink according to claim11, wherein said membrane is nanoporous or microporous.
 18. A method tocool a microelectronic device, comprising: contacting a microelectronicdevice to at least one phase change heat sink according to claim 11;providing said liquid fluid to said heat sink; absorbing heat byvaporizing said liquid fluid to said vapor fluid; passing said vaporfluid through said membrane of said heat sink; condensing vapor fluid ata condenser to reform said liquid fluid; and re-providing said liquidfluid to said heat sink, wherein the vapor quality of said vapor fluidis 100%.
 19. The method of claim 18, wherein said providing is performedwith a pump.
 20. The method of claim 18, wherein said vaporizing saidliquid fluid to said vapor fluid occurs between said fluid reservoir forsaid liquid fluid and a vapor microchannel.